The present invention relates to so-called diaphragm springs, also known as Belleville springs, which can be utilized in many types of friction clutches for automotive vehicles or the like.
A diaphragm spring of the type under consideration here comprises an annular outer section and an annulus of radially inwardly extending fingers or prongs whose inner end portions can be engaged to cause the spring to pivot about one or more seats in the general area where the fingers extend inwardly from the annular section and to thus change its shape (conicity). As a rule, the neighboring fingers of the annulus are separated from each other by elongated radially extending slots having enlarged outermost portions in the region of the annular section. A diaphragm spring can be mounted in such a way that it is flat or nearly flat in idle condition but its conicity invariably changes when in use, e.g., in order to engage or disengage a friction clutch by establishing or terminating a torque-transmitting connection between a rotary driving member and a rotary driven member. Typical examples of driving members are the flywheels of internal combustion engines, and typical examples of driven members are flanges on the input shafts of change-speed transmissions in automotive vehicles.
It is known that the annular section of a diaphragm spring of the above outlined character is not free to undergo optimum deformation in response to displacement of the tips of fingers in the axial direction of the spring. This is due to the fact that the rigidity of the transition zone between the annular section and the fingers (at least as considered in the circumferential direction of the annular section) is quite pronounced. The increased localized rigidity causes the formation of undulations, as considered in the circumferential direction of the annular section, when the diaphragm spring is installed in prestressed condition. The pronouncedness of such undulations depends on the ratio of width of the annular section to the width of the fingers in the transition zone. The undulations are more pronounced if the aforementioned ratio is reduced, i.e., if the width of the fingers with reference to the width of the annular section is increased. Furthermore, undulations of the annular section entail the development of additional tensional stresses whose magnitude increases with increasing width of the fingers and which can become critical in the region between the enlarged outermost portions of the slots between the fingers. In many instances, the stresses which arise as a result of unsatisfactory ratio of the width of the fingers to the width of the annular section lead to premature breakage of the annular section with attendant damage to or complete destruction of the device or aggregate in which the diaphragm spring is used. The consequences of such breakage can be readily appreciated if one considers that diaphragm springs are important components of a large number of friction clutches for automotive vehicles.
In view of the above, one would be led to the assumption that the drawbacks of conventional diaphragm springs can be overcome by the simple expedient of increasing the number of fingers with attendant reduction of their width in the region of the annular section and with attendant reduction of the ratio of width of the fingers to width of the annular section. An alternative or additional solution would involve increasing the width of the enlarged outermost portions of slots between neighboring fingers. However, for reasons of manufacture as well as for reasons of ensuring that the useful life of a diaphragm spring will not be reduced below a certain minimum acceptable period of time, the just discussed modifications of conventional springs can be carried out only within a relatively narrow range. As disclosed in German Utility Model No. 1,928,288, the width of an enlarged outermost portions of slots between the fingers of a diaphragm spring which is used in a friction clutch cannot be increased at will because this would necessitate the utilization of larger-diameter rivets which secure the spring to a support, such as the cover of a friction clutch. The diameters of the shanks of the just mentioned rivets must be increased if the width of the outermost portions of the slots is increased because, in addition to their connecting or securing action, the rivets also perform the function of centering the spring with reference to its support. The use of larger and heavier rivets contributes to the cost of the clutch as well as to the bulk (as considered radially and axially of the clutch) and weight of the spring and of the device in which the spring is put to use.
As concerns the number of fingers, i.e., increasing the number of fingers with attendant reduction of their width in the region of annular section of the spring, an undue increase in the number of fingers is not warranted and advisable in many instances for several reasons, such as the facility of making the spring and especially the desired useful life of the spring. For example, when a diaphragm spring is used in a friction clutch, the area of contact which is established by the inner end portions of the fingers (namely, by those portions which are to be acted upon by the clutch engaging and disengaging means, such as an annular pusher or a ball bearing) cannot be reduced below a certain value without unduly increasing the wear upon the tips of the fingers. As regards the slots between neighboring fingers, their width cannot be reduced below a certain value for manufacturing reasons. In other words, by increasing the number of fingers well beyond the number of fingers in presently known diaphragm springs, one would unduly reduce the ratio of combined area which is taken up by the slots to the combined or total area which is taken up by the fingers with attendant reduction of the aforementioned area of contact between the inner end portions of the fingers and the parts which effect the engagement or disengagement of the clutch wherein the spring is used. As mentioned above, the area of contact cannot be reduced at will because this entails a pronounced increase of wear upon the tips of the fingers.
Commonly owned French patent application Ser. No. 78 25606 discloses a pulley for a V-belt. The pulley employs a diaphragm spring with wider and narrower slots between neighboring fingers and with links which extend substantially tangentially of the hub of the pulley and are secured to fasteners which extend through the enlarged outermost portions of wider slots. The relatively wide slots contribute significantly to a reduction of the combined area of the inner end portions or tips of the fingers.
European patent application Ser. No. 0 23 790 discloses a diaphragm spring which is used in a friction clutch and wherein the flexibility of fingers forming part of the spring is increased by removing certain fingers. Here, too, the combined area of the inner end portions or tips of the fingers is unduly reduced as a result of elimination or removal of certain fingers.
German Utility Model No. 76 21 265 discloses a friction clutch which employs a diaphragm spring with radially inwardly extending fingers each flanked by a relatively wide and a relatively narrow slot (note FIG. 4). The wide slots contribute to a significant reduction of the combined area of the inner end portions or tips of the fingers so that the wear upon such finger tips is highly pronounced when the clutch is in use.